1. Field of the Invention
This invention relates to a gas-detection sensor and more particularly to a solid state mass spectrograph which is micro-machined on a semiconductor substrate, and, even more particularly, to a solid-state detector used in such a mass spectrograph.
2. Description of the Prior Art
Various devices are currently available for determining the quantity and type of molecules present in a gas sample. One such device is the mass-spectrometer.
Mass-spectrometers determine the quantity and type of molecules present in a gas sample by measuring their masses. This is accomplished by ionizing a small sample and then using electric and/or magnetic fields to find a charge-to-mass ratio of the ion. Current mass-spectrometers are bulky, bench-top sized instruments. These mass-spectrometers are heavy (100 pounds) and expensive. Their big advantage is that they can be used in any environment.
Another device used to determine the quantity and type of molecules present in a gas sample is a chemical sensor. These can be purchased for a low cost, but these sensors must be calibrated to work in a specific environment and are sensitive to a limited number of chemicals. Therefore, multiple sensors are needed in complex environments.
A need exists for a low-cost gaseous detection sensor that will work in any environment. U.S. patent application Ser. No. 08/124,873, filed Sep. 22, 1993, hereby incorporated by reference, discloses a solid state mass-spectrograph which can be implemented on a semiconductor substrate. FIG. 1 illustrates a functional diagram of such a mass-spectrograph 1. This mass-spectrograph 1 is capable of simultaneously detecting a plurality of constituents in a sample gas. This sample gas enters the spectrograph 1 through dust filter 3 which keeps particulates from clogging the gas sampling path. This sample gas then moves through a sample orifice 5 to a gas ionizer 7 where it is ionized by electron bombardment, energetic particles from nuclear decays, or in a radio frequency induced plasma. The mass filter 11 applies a strong electromagnetic field to the ion beam. Mass filters which utilize primarily magnetic fields appear to be best suited for the miniature mass-spectrograph since the required magnetic field of about 1 Tesla (10,000 gauss) is easily achieved in a compact, permanent magnet design. Ions of the sample gas that are accelerated to the same energy will describe circular paths when exposed in the mass-filter 11 to a homogenous magnetic field perpendicular to the ion's direction of travel. The radius of the arc of the path is dependent upon the ion's mass-to-charge ratio. The mass-filter 11 is preferably a Wien filter in which crossed electrostatic and magnetic fields produce a constant velocity-filtered ion beam 13 in which the ions are disbursed according to their mass/charge ratio in a dispersion plane which is in the plane of FIG. 1.
A vacuum pump 15 creates a vacuum in the mass-filter 11 to provide a collision-free environment for the ions. This vacuum is needed in order to prevent error in the ion's trajectories due to these collisions.
The mass-filtered ion beam is collected in a ion detector 17. Preferably, the ion detector 17 is a linear array of detector elements which makes possible the simultaneous detection of a plurality of the constituents of the sample gas. A microprocessor 19 analyses the detector output to determine the chemical makeup of the sampled gas using well-known algorithms which relate the velocity of the ions and their mass. The results of the analysis generated by the microprocessor 19 are provided to an output device 21 which can comprise an alarm, a local display, a transmitter and/or data storage. The display can take the form shown at 21 in FIG. 1 in which the constituents of the sample gas are identified by the lines measured in atomic mass units (AMU).
In any ionic mass spectrometer or charge sensing device, there must be some means to collect the charge and determine its magnitude. For high performance devices, sensitivity of 10's of charges at speeds of 10's of kilocycles is required. An additional resolution constraint is mandated for mass spectrographs: the detector pitch must be smaller than the ion beam while insuring that the ion beam is not missed due to interdetector spacing of non-contiguous detector elements. As detector pitch is reduced, smaller displacements (i.e., better mass resolution in a miniaturized package) can more readily be discerned.
In the present state of the art, charge multiplication devices and high gain current sensors have been utilized. Charge multiplication devices require high voltages (&gt;1000 volts) in order to operate. This is difficult to implement on a silicon chip where voltages are generally less than 100 volts. High gain current amplifiers, often referred to as electrometers, operate at low voltages and can be used to measure total charge. Electrometers typically found in laboratory instruments are useful for currents on the order of 1.times.10.sup.-14 amperes. However, this sensitivity is at the expense of speed, with response time approaching several seconds for these low current values.
Another charge sensor which is typically used for the detection of light and high energy particles is the charge-coupled device (CCD). Photoelectrons generated at a capacitor or charge injection from a high energy particle onto a capacitor are moved by the CCD to a charge sensitive amplifier and converted to a voltage signal which can be sensed. CCDs are capable of sensing low amounts of charge (some as low as 10's of charges per read cycle) with read rates in the 10's of kilocycles, but require a passivating dielectric over the charge storage capacitor to protect the active CCD semiconductor layers from environmental degradation. This dielectric precludes sensing of low energy molecular and atomic ions.
High speed and low charge sensing devices capable of accurately detecting low energy molecular and atomic ions are required to effectively miniaturize ionic gas sensors. Accordingly, there is a need for a solid-state detection for sensing low energy charge particles.